Atrial fibrillation (AF) is the most common arrhythmia in the world, and a significant source of morbidity and mortality. Unfortunately, antiarrhythmic drugs have had limited efficacy and numerous side effects. Transvenous ablation techniques require prolonged, often multiple, procedures and have had high complication rates and questionable long-term efficacy. Over the last two decades, our laboratory has developed two successful surgical approaches for the treatment of AF. The Maze and Radial procedures have the best-documented long-term results in the treatment of atrial fibrillation, with over 90% of patients remaining arrhythmia-free at 10 years. Despite their proven efficacy, few surgeons around the world actually perform these procedures, both because of their invasiveness and complexity. Moreover, these operations are far from perfect, and still result in significant morbidity in terms of postoperative atrial arrhythmias, conduction disturbances, and atrial dysfunction. Our goal in this competing renewal is to develop a less invasive procedure with more widespread applicability by moving from an anatomically based operation that was designed to eliminate all possible mechanisms of AF to a mapping-directed procedure that addresses the specific focal mechanisms responsible for AF in each particular patient. To accomplish this goal and to evaluate the physiologic consequences of our interventions, the following specific aims are proposed: 1) Develop a real-time algorithm using intraoperative epicardial mapping to identify focal sources and substrates for AF to allow for mapping-directed therapy. 2) Develop less-invasive surgical procedures for the treatment of AF based on real time mapping data and the assessment of the physiological consequences and mechanistic insight from the operations. 3) Develop techniques to evaluate atrial function non-invasively to allow for the precise assessment of the hemodynamic consequences of surgery for AF. 4) Determine the causes of postoperative arrhythmias associated with surgical procedures to ablate AF, and modify these procedures to minimize their occurrence. If we can achieve these aims, the thousands of chronic AF patients yearly who undergo valvular and coronary surgery will have their arrhythmia cured in the operating room at the time of surgery, and our surgical treatment will become a viable option to a lifetime of anti-arrhythmic drug therapy and anticoagulation for millions of patients with lone AF.